Types of Combustion

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Types of Combustion

• When the basic components for a fire, (fuel,
  oxygen and heat) are brought together in the
  right amount, an uninhibited chemical reaction
  (rapid oxidation process) occurs, which generates
  heat and sometimes light.
• One of four possible processes has occurred.
• These processes will be looked at in more detail
• To examine why fire burns in different ways and,
• Look at the differences in energy output and
  combustion products.

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Types of Combustion

• Four basic types of combustion
• Spontaneous Heating and Combustion
• Smoldering
• Diffusion
• Pre-mixed

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Spontaneous Heating and Combustion

• The process whereby a material increases in
  temperature without drawing heat from its
  surroundings. 
• Most tests we conduct and simulate begin with an
  open flame.

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Spontaneous Heating and Combustion

• In the practice of investigation we become
  complacent with thinking that most fires start
  with an open flame.
• Many fires may actually begin the combustion
  process several days, weeks and even months
  before they are actually discovered.
• What they are doing at this time is self-heating.

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Spontaneous Heating and Combustion

• It stays hidden as long as its ideal conditions
  are present and only when some sensory sign is
  observed, is it noticed.
• Examples of sensory signs include
• Strange odors
• Burning sensation on the eyes
• Haze or light smoke seen.
• Many times when the fire is discovered early, we
  find indicators that the self-heating process
  actually began at some time in the past until the
  conditions were present for the combustion
  process to occur.

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Spontaneous Heating and Combustion

• There are many terms that relate to the concept
  of self-heating and spontaneous combustion.
• Several terms have evolved over the years as we
  learn more about the process.
• Quintiere, J.G. Principles of Fire Behavior.
• Spontaneous Combustion is a process by which
  combustion occurs after a self-incubation period
  between a fuel and oxidizer.
• DeHaan, J.D. Kirks Fire Investigation 5th
  Edition.
• The term Spontaneous chemical causation would be
  more accurate, for in such cases the mass of
  fuel, rather than igniting from one point or even
  one surface, ignites throughout its mass at very
  nearly the same time, while the heat has been
  accumulating not from some exterior source, but
  from chemical processes occurring within the fuel
  mass.

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Spontaneous Heating and Combustion

• Even though these are different terms, all agree
  that it is a combustion process that begins with
  a slow oxidation of a fuel exposed to air.
• This slow process is dependent on the fuels
  ability to retain heat.
• If this heat cannot dissipate, the temperature of
  the fuel itself will increase causing a faster
  chemical reaction and may eventually evolve into
  smoldering or flaming combustion.
• The time of this process, from the initial
  self-heating, until the final stages of
  combustion, may be a few hours to over many weeks.

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Spontaneous Heating and Combustion

• Spontaneous Combustion can occur in most organic
  and certain inorganic materials.
• The key elements being
• Presents of a drying oil (vegetable or animal in
  origin)
• A porous support medium such as a cotton cloth
• Time for the reaction to occur.
• Hydrocarbon oils and animal lard are not included
  in this phenomenon and spontaneous combustion of
  these products will not occur.

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Spontaneous Heating and Combustion

• Conditions required for the process
• Three factors will control the combustion process
  once all of the previous elements are satisfied.
• 1.      Rate of heat generation.
• The overall process is slow, however the rate of
  heat generation by the material must be faster
  than the rate heat is transferred into its
  surroundings for it to cause ignition.
• Since reaction rate doubles for every 10C
  increase in temperature, heat production must
  stay within a localized volume reacting material.
  

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Spontaneous Heating and Combustion

• Conditions required for the process
• 2.      Effects of ventilation.
• A quantity of air must be present to permit
  oxidation, but too much air will dissipate heat
  through convection.
• 3. Insulating effects of the materials immediate
  surroundings.
• The objects or products that surround a material
  that is prone to spontaneous combustion must have
  an insulating effect to the material in order for
  it to retain heat.

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Spontaneous Heating and Combustion

• Other factors occur such as the higher the
  ambient temperature and higher the relative
  humidity, the faster the reaction and the larger
  the fuel array the more it is prone to
  spontaneously combust.
•  
• Example
• Linseed oil in itself is not subject to
  spontaneous combustion or self-heating and
  self-ignition.
• However, if a light coating is present on a
  cotton cloth, there will be enough surface area
  to promote oxidation and heating.

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Spontaneous Heating and Combustion

• Example
• If the cloth were spread out over an object or
  hung on a line, the convective air would
  dissipate the heat and no ignition would occur.
• If this same cloth were crumpled and put in a
  container with other materials, it would begin
  its self-heating process.
• If the proper amount of air is present and the
  other materials insulated the cloth sufficiently,
  it would continue to heat until it reached the
  ignition temperature of the cloth or surrounding
  materials and ignite them.

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Smoldering

• Combustion without flame, usually with
  incandescence and smoke. 
• Smoldering is a relatively slow combustion
  process that occurs between oxygen in the air and
  a solid fuel.
• No flame is present, however the presence of very
  hot materials is on the surface of which
  combustion is proceeding.

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Smoldering

• The surface undergoes glowing and charring.
• The glowing is indicative of a temperature in
  excess of 1000C.
• A smoldering or glowing condition can occur at
  any point in the fire with the controlling factor
  being ventilation.

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Smoldering

• Smoldering during the initial stage of the fire
• All structures, objects, and open areas for the
  most part, have a fuel and an oxygen source
  available, and in many locations within, the fuel
  and oxygen are in the ideal proportion to begin
  reacting.
• Even though everything is ready, the combustion
  process cannot begin until some source of heat
  triggers the process.

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Smoldering

• Smoldering during the initial stage of the fire
• Because this process is slow, not much air
  (oxygen) is required, even when the surrounding
  environment is at a relatively low temperature.
• The hotter the environment, the less oxygen it
  takes to sustain smoldering.
• The replacement of air takes place naturally as
  the air cycles.

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Smoldering

• Smoldering during the initial stage of the fire
• During the smoldering process the oxygen and fuel
  are not in a balanced condition, thus its
  by-products are the result on incomplete burning.
• High levels of carbon monoxide (CO) are produced.
• More than 10 of the fuel mass is converted to CO.

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Smoldering

• Smoldering during the initial stage of the fire
• The smoke from this type fire tends to condense
  on walls, windows and other cooler surfaces.
• These deposits of pyrolysis products will be
  widely distributed and the colors vary in shades
  of brown.
• The appearance can be sticky or wet, thick or
  dry, or dried and resinous.

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Smoldering
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Smoldering

• Smoldering during and post fire
• During a flaming fire, conditions may exist that
  decrease the oxygen level below 16.
• This causes the combustion process to slow, the
  flames subside and the temperature begins to
  decrease.
• Smoldering combustion will continue because of
  the very high temperatures at the oxidizing
  surface.

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Smoldering

• Smoldering during and post fire
• As long as these temperatures remain high, the
  process can continue even in an oxygen depleted
  atmospheres where the oxygen levels are as low as
  approximately 5.
• Even in post-fire conditions, after the fire
  department extinguishes the fire with water,
  hidden areas where the water doesnt reach will
  still smolder until the water penetrates it or
  until it consumes the fuel source.

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Smoldering

• Smoldering during and post fire
• In a room or structure where inadequate
  ventilation exists, the buildup of carbon
  monoxide and other fire gases resulting from
  incomplete combustion, cannot escape the confines
  of the area.
• Nor can it draw in the necessary quantities of
  fresh air to sustain open burning.
• When this occurs, the area is very vulnerable to
  a phenomenon known as a backdraft or flashback or
  smoke explosion.

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Smoldering

• Smoldering during and post fire
• This occurs when a supply of fresh air is
  introduced to the area.
• Examples of sources can be from
• Windows falling out from the heat and/or pressure
  buildup,
• Persons or firemen opening up doors or breaking
  windows.
• If this type condition exists it will return to a
  free burning fire unless extinguished.
• Even after the fire has been suppressed and all
  visible flames have been extinguished, a
  deep-seated smoldering fire can again return to
  open flame if all the deep-seated areas where
  smoldering occurs are not uncovered and
  extinguished.

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Diffusion Flames

• A DIFFUSION FLAME is a combustion process in
  which the fuel gas and oxygen are transported
  into the reaction zone due to concentration
  differences.
• The vast majority of un-wanted and un-controlled
  fires (i.e. the fires you investigate) are
  diffusion flames.
• A fuel gas and the oxygen in air will move into
  the reaction area similar to the diffusion
  process in which a drop of food coloring does
  when put into a glass of water.

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Diffusion Flames
Diffusion
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Diffusion Flames

• The combustion zone is where the fuel gas and
  oxygen are in correct proportion and it has heat
  to sustain the uninhibited chemical reaction
  which is emitting heat and light.
• This zone is some distance from the fuel surface.
  The region between the fuel surface and the
  combustion zone is too fuel rich to burn.

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Diffusion Flames

• Oxygen diffuses toward the fuel surface and the
  fuel gas diffuses away from the fuel surface.
• The resulting combustion products and heat
  diffuse away from the flame zone or area.

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Diffusion Flames
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Diffusion Flames

• Once ignition occurs the rate of fuel generation
  needs to continue or increase in order to
  maintain the flame.
• The rate of fuel generation will be maintained as
  long as the convective and radiative heat
  feedback continues to decompose the fuel
  (pyrolysis).
• A laminar diffusion flame typically appears
  smooth and steady like a candle or match flame.

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Diffusion Flames

• Most fires have a larger base and a longer flame
  height than the match or the candle, as the hot
  gases rise pushing the denser cool air aside.
• After the flame height grows 12 to 20 above the
  fuel surface, it begins to search for more fuel
  and oxidant.
• It will become instable and wiggle randomly.

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Diffusion Flames

• When this occurs it is called a turbulent
  diffusion flame.
• While a flame is considered in a laminar state,
  convective heat transfer from the flame to the
  solid fuel surface controls the burning rate.

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Diffusion Flames

• The oxidant travels toward the flame induced by
  the upward flow of the buoyant hot combustion
  products.
• The forward heat transfer from the flame to the
  uninvolved fuel governs the spread rate.
• When the flame becomes turbulent, radiation
  becomes the dominant form of heat transfer.

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Diffusion Flames
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Pre-Mixed Flames

• PRE-MIXED FLAME - a flame for which the fuel and
  oxidizer are mixed prior to combustion, as in a
  laboratory Bunsen burner or a gas cooking range
  propagation of the flame is governed by the
  interaction between flow rate, transport
  processes and chemical reaction.
• Other examples of pre-mixed flames are
• An oxyacetylene torch
• Methane leak, and
• Gasoline engine

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Pre-Mixed Flames

• Every fuel gas has an upper (UFL) and lower (LFL)
  flammable limit concentration in which flame
  propagation can occur.
• Most charts and tables of the various fuel gases
  and their flammable or explosive limits are
  calculated at 25C in air at a normal pressure.

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Pre-Mixed Flames

• These limits vary slightly with temperature
  changes as noted in the following figure.
• The diagram shows that the temperature of the
  mixture increases until it reaches the
  auto-ignition temperature (AIT).

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Pre-Mixed Flames

• When the fuel to air ratio is ideal or
  stoichiometric and the temperature is near the
  point where the fuel will condense, a small
  energy source may initiate sustained propagation.

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Pre-Mixed Flames

• An example of a small energy source (less than
  1mJ) would be like a spark or electric discharge.
• As the temperature rises toward the fuels
  autoignition temperature, the fuel to air mixture
  can ignite without any energy source.

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Pre-Mixed Flames

• While a fuel to air mixture is in this ideal
  concentration the speed of flame propagation can
  be less than 1 m/s and if it is in a confined
  space it will manifest itself as an explosion
  with a shock wave.
• Combustion velocities can be faster than the
  speed of sound when in the middle of the
  flammable range.
• When this occurs it is called a detonation.